This invention relates to tonometry and, in particular, to a non-contact tonometer for determining intraocular pressure and thus the health of a human eye.
Tonometry broadly relates to measuring tension in living tissue and has come to have special meaning in ophthalmology where it relates more specifically to intraocular pressure from which the health of the eye can be determined. Pressure within a living eye cannot be measured directly and has heretofore been inferred by measuring the eyes response to some external force. One of the most widely employed test in the art is the Goldmann applanation test wherein the cornea of the eye is flattened by a probe having a planar contact surface. The cornea is depressed until such time as a given radius is formed at the mouth of the depression. The force required to produce this depression is measured and intraocular pressure is inferred from the measurement. By testing a large number of subjects over a relatively long period of time, reasonable standards have been established so that diseased or glaucomatous eyes are now readily identified.
The Goldmann applanation test, and other similar tests, require that the test instrument physically touch and deform the cornea thereby increasing the risk of infection and the possibility of the cornea becoming abraded or otherwise damaged. Because of the risk involved, these contact tests must be conducted by a physician or a trained clinical technician. It is also common practice to anesthetize the patient when carrying out these contact tests in order to minimize patient discomfort and unwanted head movement. The use of anesthesia, however, sometimes produces an adverse effect in the patient and, because the patient is unable to feel normal pain, actually increases the risk of the damage.
The reliability of applanation test results are sometimes adversely effected by a number of uncontrollable variables. These include the size and shape of the patients eye, the amount of aqueous humor escaping from the eye as the cornea is being compressed, variations in the response of the sclera to the applied pressures, and the unwanted movement of the patient's head during testing. Attempts to correct or compensate test results for these uncontrolled variables have met with only limited sucess. Furthermore, because the applanation test requires that the instrument physically contact and depress the cornea, potentially harmful micro organisms can enter the patient's body through eye fluids if the contacting surface of the instrument is not carefully cleansed.
Lechtenstein, et al. in U.S. Pat. No. 3,545,260 discloses an eye test wherein corneal compliance is used to describe intraocular pressure. In this particular test, the eye is sealed within a gas filled chamber and the gas pressurized to a level sufficient to deform the cornea. The depth of depression created by the gas is measured using energy waves. Here again, the cornea of the eye must be deformed to obtain an indication of its health. Accordingly, the test poses a certain amount of danger and can only be repeated at safe intervals. In addition, it is extremely difficult to obtain and maintain a tight gas seal around the eye. Failure of the seal produces erroneous test results.
A later issued Lechtenstein et al. Pat. No. 3,690,158 discloses another test in which acoustical energy is used to measure the impedance of a patient's eye and thus provide information concerning intraocular pressure. Acoustical waves are directed through a liquid medium at both the patient's eye and a dummy target that has been arranged to simulate the behavior of a healthy eye. The acoustical impedance of the patient's cornea is measured by comparing the energy it reflects with the energy reflected by the dummy target through the liquid medium. The two impedances are then compared and any slight differences between the two provides indication as to condition of the eye. It is extremely difficult to keep a human eye sealed within the liquid medium and any movement or displacement of liquid will have an adverse effect on the test results. Accordingly, this type of testing has not found wide acceptance in the industry.